Cation-Controlled Electrocatalytical Activity of Transition-Metal

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Cation-Controlled Electrocatalytical Activity of Transition Metal Disulfides Jan Luxa, Pavel Vosecký, Vlastimil Mazánek, David Sedmidubsky, Martin Pumera, and Zdenek Sofer ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04233 • Publication Date (Web): 16 Feb 2018 Downloaded from http://pubs.acs.org on February 17, 2018

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Cation-Controlled Electrocatalytical Activity of Transition Metal Disulfides Jan Luxaa, Pavel Voseckýa, Vlastimil Mazáneka, David Sedmidubskýa, Martin Pumeraa,b, and Zdeněk. Sofera* a

Department of Inorganic Chemistry, University of Chemistry and Technology Prague,

Technická 5, 166 28 Prague 6, Czech Republic. E-mail: [email protected]; Fax: +42022044-0422 b

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical

Sciences, Nanyang Technological University Singapore 637371, Singapore

ABSTRACT The nanostructured layered transition metal dichalcogenides are highly perspective materials for substitution of platinum metals in electrocatalysis. These layered materials are usually exfoliated with intercalation of Li from organolithium compounds. Alkali metal intercalation leads not only to exfoliation and increased surface area but also, even more importantly, to conversion of naturally occurring 2H (trigonal prismatic) semiconducting phase of MoS2/WS2 to 1T octahedral conducting phase which also exhibits better electrocatalytical performance. Surprisingly, even for the most broadly studied members like MoS2 and WS2, systematic work on their exfoliation is missing. We present a detailed study of MoS2 and WS2 exfoliation by intercalating cations of variable size (Li, Na, K, Rb, Cs). Alkali metal naphtalenides which are capable of producing single layer exfoliated dichalcogenides in quantitative yield were used. The results show strong dependence of cation ionic radii on dichalcogenide exfoliation. The cation has also a significant influence on the 1T/2H phase ratio in the resulting exfoliated

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materials. The overpotential and Tafel slopes directly correlate with concentration of 1T phase. Our findings have broad fundamental implications as well as practical applications as by choosing the intercalating ion, one can tailor the exfoliation procedure. KEYWORDS hydrogen evolution reaction; transition metal dichalcogenides; intercalation; alkali metal

INTRODUCTION With the continuously growing population on Earth there is also an increasing demand for renewable energy sources. Hydrogen economy offers an economical and environmentally friendly solution. Its great advantage stems from high energy density as well as harmless byproducts. Hydrogen evolution reaction (HER) water splitting seems to be an optimal way of producing hydrogen, however, high overpotential of this reaction requires an efficient catalyst. Currently, noble metals such as platinum belong to the top-tier group of HER catalysts. Disadvantages of using such catalysts are evident: rarity and price. For this reason, cheaper alternatives with similar or superior performance are highly desirable. The aforementioned reasons indicate that enormous amount of research has to be devoted to the study of HER catalysts. Layered materials have emerged as a viable competition for platinum. Ever since the discovery of graphene, layered materials have been gaining a renewed interest. In terms of HER activity, layered materials like transition metal dichalcogenides (TMDs),1-7 transition metal phosphides (TMPs),8-10 transition metal carbides (TMCs)11-13 and others14-16 have proved to be the most promising. The low cost and intrinsic high catalytic activity of TMDs, especially that of Group 6 (MoS2, WS2), make them particularly attractive for the scientific community as well as for the application sphere. These materials consist of stacked Van der Waals bonded layers offering the possibility of exfoliation down to monolayer thickness.17 As a consequence, new properties emerge due to


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quantum confinement effects.4 Transition from indirect to direct band-gap18-19 and the appearance of photoluminescence are examples of these phenomena. Other devices related to energy generation and storage such as supercapacitors,20-21 batteries22-23 and solar cells24 have also been reported. Edge sites are generally recognized as HER active sites in TMDs,25-27 therefore, one of the easiest and efficient ways of increasing HER activity is to simply increase the edge site concentration. To achieve this, appropriate exfoliation techniques such as liquid-phase exfoliation or chemical exfoliation have to be employed. These techniques also belong to the most commonly used.28-30 Whereas liquid-phase exfoliation uses appropriate solvent or combination of solvent and surfactant,31 chemical exfoliation utilizes organometallic compounds such as n-butyllithium.32 Both methods can yield sheets of single-layer thickness, however, chemical exfoliation also induces a transition from 2H phase to 1T phase.18,

33

During the phase transition, there is a change from prismatic to octahedral coordination of the metal atoms accompanied by a significant enhancement of electrical conductivity. Moreover, basal plane of 1T phase is HER active which further increases the overall catalytic activity.34 Herein we report a comprehensive study using naphtalenide salts of alkali metals (Li, Na, K, Rb and Cs) for the exfoliation of MoS2 and WS2. An extensive characterization has revealed that cation size is the dominating factor in the degree of exfoliation and 2H to 1T phase transition. Whereas sodium naphtalenide has been identified as the most efficient exfoliation reagent for WS2, lithium naphtalenide turned out to be the best for MoS2. The developed exfoliation procedure produce highly catalytic MoS2 and WS2 exfoliated down to the single layer in nearly quantitative yield.


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RESULTS AND DISCUSSION We investigated the influence of using various alkali metal cations on the electrocatalytical properties of MoS2 and WS2. Naphtalenide salts in tetrahydrofuran (THF) were used for this purpose. First, we investigated the composition and structure by various analytical techniques including scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS), X-ray powder diffraction (XRD), Raman spectroscopy and Xray photoelectron spectroscopy. Electrochemical properties were assessed by measurement of hydrogen evolution reaction (HER) activity. We studied the morphology of exfoliated samples by SEM with the results shown in Figure 1. Generally, we observed a higher degree of edge wrinkling and smaller crystallites for lighter cations (Li, Na). This is especially apparent for MoS2 while this effect is not so clearly identified for WS2. Therefore, we anticipated a higher degree of exfoliation and the related HER activity for light cations exfoliated TMDs. This was confirmed and is discussed in the following paragraphs. EDS analysis revealed much higher oxygen content in MoS2 Li and MoS2 Na samples. We assigned this to high degree of exfoliation which dramatically increased the surface area and enhanced the rate of oxidation.32 The remainder of samples exhibited much lower oxygen concentration not exceeding 6.2 wt.%. The results are summarized in Table S1.

Figure 1. SEM images of exfoliated MoS2 and WS2. Scale bars correspond to 5 µm.


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We also performed scanning transmission electron microscopy for getting further structural information. Images in Figure S1 illustrate the previously mentioned effectiveness of MoS2 Li and MoS2 Na exfoliation. Sub-micron crystallites with low thickness were observed which is in high contrast with the rest of the samples composed of thicker and larger flakes. The lower degree of WS2 exfoliation is also highlighted in the respective images. Thicker nontransparent sheets are mostly visible in this case. We further used X-ray powder diffraction (XRD) method to verify the phase purity and crystallinity of the exfoliated TMDs. Our results, displayed in Figure 2 and summarized in Table S2, concur with SEM observations. Samples exfoliated with heavier cations (K. Rb and Cs) have sharp intense reflections indicating insignificant changes in crystallinity, therefore, low degree of exfoliation. On the other hand, Li and Na exfoliated MoS2 show no reflections at all. This suggests a complete exfoliation down to single layer material. While this feature was not observed for Li and Na exfoliated WS2 they still exhibited significantly broader peaks. Notably, no oxide peaks were identified even in diffractograms of oxidized MoS2 samples indicating that the products of oxidation, if present, are of amorphous character. Since the interlayer distances are almost identical in MoS2 and WS2, the lower degree of exfoliation observed in general for all WS2 samples must originate from its chemical properties and intercalation abilities.


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Figure 2. X-ray diffractograms of exfoliated MoS2 and WS2.


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Raman spectroscopy, a powerful characterization tool capable of determining some key properties such as doping or strain in TMDs,35-36 was further employed. Two dominant phonon modes are featured in Raman spectra of the exfoliated MoS2 in Figure 3a. Denoted as A1g and E12g, these modes originate from out-of-plane and in-plane vibrations, respectively.3738

The peak separation value is dependent on the number of layers, thus it can provide further

information about the degree of exfoliation.37, 39 Peak separation of about 21.2 cm-1 and 21.7 cm-1 for MoS2 Li and MoS2 Na, respectively, confirmed the complete exfoliation down to a single layer. The peak separation values are in perfect agreement with the values observed on single layer MoS2.39 Other MoS2 (K, Rb and Cs) samples show peak separation of ~26 cm-1 which is a typical value for multilayered MoS2.39 Additionally, peak at ca. 330 cm-1 in the spectrum of MoS2 Li originates from 1T phase of MoS2. The high concentration of 1T phase was further confirmed by XPS. Raman spectra over wider range of wavelengths are shown in the Supporting information (Figure S2). These spectra clearly demonstrate that there are dominant peaks originating from 1T phase in the spectrum of MoS2 Li. Notation J1, J2, J3 and E1g for 1T MoS2 active phonon modes is used in accordance with previous reports.40 The highest concentration of 1T phase in MoS2 Li was also confirmed by XPS. The peak characteristic of MoO3, most obvious in the spectrum MoS2 Na, is also highlighted.41 Since no molybdenum oxides were observed by X-ray diffraction, molybdenum oxides are present in the amorphous form or as a single layer formed by topochemical transformation of MoS2 to MoO3.


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Figure 3. Raman spectra of the exfoliated a) MoS2 and b) WS2.

Figure 3b displays Raman spectra of the exfoliated WS2 samples. In addition to peaks similar to MoS2, additional disorder induced 2LA mode is present in the spectrum.42 The ratio of the two peaks can serve as an indication of the number of layers. The intensity ratio of A1g/(2LA+E12g) in the bulk WS2 is commonly >2,42 whereas in exfoliated materials this ratio falls down below MoS2 Rb > MoS2 Na. The worst performance of MoS2 Na sample was attributed to its very high degree of oxidation lowering the concentration of HER active edge sites (sulfided molybdenum edge25) as well as to a low content of 1T phase. Furthermore, as a smaller cation, rubidium was expected to more efficiently exfoliate MoS2 and result in more efficient HER catalyst. Our results suggest that caesium is slightly more efficient, however, the difference in overpotential is very small (ca. 30 mV). The content of 1T phase was also very similar for both samples, therefore, the overall HER performance of these two samples should be considered as more or less the same. A very similar trend in overall HER performance was observed for WS2 samples. The trend observed here is WS2 Na > WS2 Li > WS2 K > WS2 Rb > WS2 Cs. WS2 Na with an overpotential of about 0.41 V outperformed its lithium exfoliated counterpart. The highest content of 1T phase (XPS) was identified as the reason for this behavior. Similarly to MoS2, rubidium and caesium exfoliated WS2 exhibited very similar performance. The degree of exfoliation and 2H to 1T phase transition seems to be limited and similar for these two elements. We also evaluated Tafel slope as another marker of HER performance. Figure 6 illustrates that the tafel slope values correspond with the overall HER performance quite well. The Tafel slope of 48 mV/dec for MoS2 Li is particularly notable. This low value lies very close to that of Pt/C catalyst used here (38 mV/dec) and demonstrates the superior performance of this catalyst.


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Figure 6. Tafel slope curves extracted from LSV curves of a) MoS2 and c) WS2. Corresponding column graphs of b) MoS2 and d) WS2 are based on three separate measurements.

The Tafel slope values of the other MoS2 catalysts are all below 100 mV/dec and follow the order described in the previous sections. WS2 samples, on the other hand, exhibit much higher Tafel slopes and only WS2 Na sample (90mV/dec) reached the Tafel slope lower than 100 mV/dec. We can conclude that the general trend of decreasing HER performance with increasing cation size is reflected in the Tafel slope for both, MoS2 and WS2. The observed trends can be explained as follows. It has been previously reported that interlayer spacing in the intecalated MoS2 increases in the order KxMoS2 >NaxMoS2 > LixMoS2, however, after solvation with water the order changes to Nax(H2O)yMoS2 > Kx(H2O)yMoS2 > Lix(H2O)yMoS2.46 This leads to bigger interlayer separation and thus to a higher degree of exfoliation of sodium intercalated MoS2. The lower degree of potassium,


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rubidium and caesium exfoliation observed in our work can be interpreted by the differences in the synthesis procedure. Zheng et. al. used pre-expansion procedure by hydrazine leading to a larger interlayer separation and better accommodation of large potassium ion. Our results show that this represents an even more limiting factor for larger rubidium and caesium ions. The high degree of our oxidation of sodium exfoliated MoS2 can be ascribed to the fact that, unlike Zheng et. al., we used high sonication power which could have led to a higher degree of fragmentation and thus to a faster oxidation rate of this material. The degree of 2H to 1T phase transition seems to be influenced by the degree of intercalation, but other factors have to be considered as well. 1T phase is stabilized by the charge transfer during the intercalation, however, as the material is exfoliated the alkali metal is deintercalated and the negative charge has to be compensated by other species. It has been reported that protons in water bilayer surrounding the sheets stabilize the negative charge.47 As a matter of fact, protons are formed upon hydrolysis and the degree of hydrolysis is more pronounced for smaller cations (higher charge to radius ratio). For this reason, smaller cations are capable of stabiliting greater residual negative charge and thus the 1T phase. Differences between MoS2 and WS2 remain questionable but they seem to be an intrinsic property of these two compounds (lower degree of exfoliation of WS2 as documented by XRD).

CONCLUSION We investigated the influence of using various alkali metals naphtalenides on the exfoliation of two transition metal dichalcogenides: MoS2 and WS2. Structural and chemical analyses have revealed that smaller cations (especially lithium and sodium) are highly efficient for the exfoliation of both materials. This effect was more apparent for MoS2 exfoliated by lithium and sodium whose X-ray diffraction patterns revealed a complete exfoliation of these materials. Furthermore, much greater extent of 2H to 1T phase transition


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was indicated for lighter elements by both Raman spectroscopy and X-ray photoelectron spectroscopy. In case of MoS2, lithium was found to be the most efficient for this purpose whereas sodium was the most efficient in case of WS2. A low concentration of 1T phase and very high degree of oxidation was detected for sodium naphtalenide exfoliated MoS2. The content of 1T phase was as high as 41.9 at. % for MoS2 exfoliated with lithium naphtalenide and 43.0 at. % for WS2 exfoliated with sodium naphtalenide. The overall hydrogen evolution reaction performance was found to correlate with the content of 1T phase very well. In general, samples with the highest 1T phase content exhibited the lowest overpotentials at 10 mA.cm-2 and the lowest Tafel slopes. Lithium exfoliated MoS2, the best performing catalyst in this work, achieved overpotential of –0.24 V and Tafel slope of 48 mV/dec. Our findings show that highly efficient catalysts with high content of 1T phase can be easily prepared by simple synthetic route.

EXPERIMENTAL Materials Granulated sulfur (99.999%), molybdenum powder (99.95%, -100 mesh) and tungsten powder (99.9%, -100 mesh) were obtained from STREM, USA. Tetrahydrofurane (THF) was delivered by Lach-Ner,

Czech Republic. THF was dried by distillation from

sodium/benzophenone mixture under argon atmosphere before use. Li, Na, K, Rb and Cs were purchased from ABCr, Germany.

Synthesis procedures The transition metal and chalcogen were weighted in stoichiometric amount with an accuracy of 1 mg to form total amount of 10 g of transition metal dichalcogenides. Extra 30 miligrams


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of chalcogen were added to the ampoule to ensure the excess of chalcogen in the reaction mixture. The metal and chalcogen were placed in a quartz glass ampoule of outer dimensions 30 x 130 mm and wall thickness of 1.5 - 2 mm. During the filling of the ampoule with metal, the contact of metal with quartz glass section used for sealing was avoided, because this can lead to degradation of quartz glass during melt sealing and subsequent rapture of the ampoule. The wall thickness should be minimally 1.3 mm to avoid ampoule rupture during thermal treatment. The ampoule was evacuated on the base pressure below 5x10-5 mbar using a pump. Subsequently the reaction mixture was heated for about 2 minutes using heat gun (300 °C). Finally the ampoules were melt-sealed using oxygen-hydrogen torch. Subsequently, the sealed ampoules were placed in a muffle furnace horizontally. Firstly, the ampoules were heated to 425 °C for 24 hours and subsequently the temperature was increased from 425 °C to 500 °C over a period of 24 hours and finally cooled down to room temperature. The content of ampoule was intensively homogenized for 10 minutes and heated to 600 °C for 48 hours. The heating and cooling rates were 5 K.min-1. This was followed by another mechanical homogenization for 10 minutes and heating to 800 °C for 48 hours. Finally the temperature was increased to 850 °C for 12 hours dwell time. Heating and cooling rates were 5 K.min-1 in all steps. Last heating step to 700 °C for 72 hours was used for MoS2 in order to avoid formation of 3R-MoS2. The excess of chalcogenide was removed by heating one side of the ampoule containing formed TMD to 600 °C for 30 minutes, while the opposite side of the ampoule was kept at room temperature. 2g of either MoS2 or WS2 were placed in a glass flask and transferred into a glove-box. Naphtalene and the corresponding alkali metal (both 1.5 molar excess compared to TMD) were added and dissolved in 50 mL of dried tetrahydrofurane. The mixture was then stirred for one week under inert atmosphere. After this the mixture was filtered and washed with dried THF several times. 50 mL of deionized water were then added under argon atmosphere.


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After 30m of sonication in water, the samples were dialyzed for several days and finally centrifuged. Samples were then dried in vacuum oven for 48h and used.

Characterizations The morphology was investigated using scanning electron microscopy (SEM) with a FEG electron source (Tescan Lyra dual beam microscope). Elemental composition and mapping were performed using an energy dispersive spectroscopy (EDS) analyzer (X-MaxN) with a 20 mm2 SDD detector (Oxford instruments) and AZtecEnergy software. To conduct the measurements, the samples were placed on a carbon conductive tape. SEM and SEM-EDS measurements were carried out using a 15 kV electron beam. X-ray powder diffraction data were collected at room temperature on Bruker D8 Discoverer (Bruker, Germany) powder diffractometer with parafocusing Bragg–Brentano geometry using CuKα radiation (λ = 0.15418 nm, U = 40 kV, I = 40 mA). Data were scanned over the angular range 5–90° (2θ) with a step size of 0.019° (2θ). Data evaluation was performed in the software package EVA. inVia Raman microscope (Renishaw, England) in backscattering geometry with CCD detector was used for Raman spectroscopy. DPSS laser (532 nm, 50 mW) with applied power of 5 mW and 50x magnification objective was used for the measurement. The samples were suspended in N,N-dimethylformamide (2 mg/ml) and ultrasonicated for 10 min. The suspension was deposited on a small piece of silicon wafer and dried. High resolution X-ray photoelectron spectroscopy (XPS) was performed using an ESCAProbeP spectrometer (Omicron Nanotechnology Ltd, Germany) with a monochromatic aluminium X-ray radiation source (1486.7 eV). Wide-scan surveys of all elements were performed, with subsequent high-resolution scans of Mo 3d, W 4f and S 2p.


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The electrochemical characterization was performed on an Autolab PGSTAT 204 (Metroohm, Switzerland). Glassy carbon electrode was cleaned by polishing with an alumina suspension to renew the electrode surface then washed and wiped dry prior to any use. Glassy carbon electrode, glass carbon rod and saturated Ag/AgCl electrode were used as working, auxiliary and reference electrodes. The samples were dispersed in N,N-dimethylformamide to obtain 2 mg/mL suspension. The suspension was then sonicated for 5 min at room temperature before every use. A cleaned GC electrode was then modified by coating with a 1 µL aliquot of the suspension and left to dry. Characterization by linear sweep voltammetry was performed in 0.5M H2SO4 solution with a scan rate of 2 mV/s. Measurements were performed three times with a new portion of material every time.

ASSOCIATED CONTENT Supporting information. The TEM images of exfoliated MoS2 and WS2 and Raman spectra of exfoliated MoS2 and WS2. AUTHOR INFORMATION Corresponding Author *Zdeněk Sofer, Dept. of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic. E-mail: [email protected].

ACKNOWLEDGMENT


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Project was supported by Czech Science Foundation (GACR No. 17-11456S and GACR No. 16-05167S) and by specific university research (MSMT No. 20-SVV/2017). This work was created with the financial support of the Neuron Foundation for science support. This work was

supported

by

the

project

Advanced

Functional

Nanorobots

(reg.

No.

CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR). Authors thank for the support of Tier 1 (99/13) and Tier 1 (123/16) from MOE Singapore, and A*Star SERC A1783c0005 grant. This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its CREATE programme.

AUTHOR CONTRIBUTIONS J.L. and P.V. performed the exfoliation and characterization experiments. D.S. and M.P. performed analysis and data evaluations. Z.S. performed the synthesis and designed the experiments. All authors wrote the manuscript.

COMPETITING FINANTIAL INTERESTS The authors declare no competing financial interests.


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